Constant-On-Time control is widely used in power supplies due to its outstanding performance of load transient response, simple circuit configuration and smooth transition between operation modes, etc.
A switching converter with Constant-On-Time control comprises an output capacitor which may be equivalent to a resistor and an ideal capacitor coupled serially together. When the resistance of the equivalent resistor is small, the switching converter may oscillate easily, and a ramp compensation signal is consequently required to stabilize the switching converter.
A prior art method employs a saw-tooth signal as the ramp compensation signal. The saw-tooth signal is in-phase with a current flowing through an output inductor of the switching converter and has a fixed magnitude which is larger than the capacitor voltage ripple of the ideal capacitor. As a result, the switching converter is stable in steady state. However, when a load of the switching converter steps up, the capacitor voltage ripple increases rapidly and the saw-tooth signal with the fixed magnitude thus fails to override the capacitor voltage ripple. The oscillation event may still occur in the switching converter.
Another prior art method uses a low-side current detection circuit to detect a current flowing through a low-side switch of the switching converter and thereby generates the ramp compensation signal. The magnitude of the ramp compensation signal varies along with the inductor current automatically when the load steps up. Thus, this method is capable of overriding the capacitor voltage ripple to make the switching converter stable. However, a high-speed current detection amplifier is usually required to realize real-time inductor current detection, which in turn complicates the circuit configuration.
In addition, the prior-art compensation circuit brings in a direct current offset in an output signal of the switching converter. Accordingly, a DC calibration circuit implemented by an error amplifier is required to cancel the DC offset. Due to the slow response of the error amplifier, it may take a long time for the output signal to get back to a preset value during load transients. Thus, the transient response of the switching converter is poor.